Final answer:
Steel begins to fail at around 700 degrees Celsius, when it loses about 50% of its yield strength, which affects its structural integrity in fire conditions.
Step-by-step explanation:
Steel starts to fail around 700 degrees Celsius (option c). This temperature is not an absolute point of failure but indicates the beginning of a reduction in the strength and stiffness of steel, which compromises its structural integrity.
At around 700 degrees Celsius, steel loses about 50% of its yield strength, which is a critical consideration in the design of buildings and structures to ensure they remain safe in the event of a fire.